1. Field of the Invention
This invention relates to voltage conversion circuits and more particularly to protecting such circuits from damage due to overload.
2. Description of the Prior Art
It is known to utilize a transformer in a voltage converter receiving a direct current d.c. voltage. The transformer in such a converter comprises a primary winding and an appropriate secondary winding and will have one or more switching transistors coupled to the primary winding. This switching transistor or transistors are switched on by control signals from a source such as a control signal source to switch an input voltage to the primary winding. In duty cycle control power converters or switching regulators of this type it is common to employ overcurrent circuits which shut off the switching element if either the peak or the average current drawn by the power supply becomes excessive endangering the switching elements or other components. A sample of the converter/switching regulator input current is obtained, often by resistor sampling or by use of a current transformer. This current sample will take the form of a pulse train, the height of the pulses of which provide a voltage analog of the sampled current. The current pulses may be rectangular, sawtooth or of any other shape required by the converter circuit and the load characteristics thereof. This voltage analog may further be low pass filtered to obtain an average voltage or used directly and compared with a reference voltage level in order to determine if over-current has occurred.
According to the prior art, a fixed reference level is used to make this comparison. In flyback or resonant converters, and in forward converters in particular, this may not provide adequate overcurrent protection since wide variations of input line voltage will result in wide variations of peak and average primary current levels drawn under normal operating conditions. Setting the overcurrent reference level above the maximum normal current which occurs at low line voltage results in very high power dissipation in the converter elements if an overcurrent condition occurs at high line voltage. Since the fixed overcurrent reference is usually built into common regulator integrated circuits, it is difficult to alter.
The present invention provides an apparatus for providing an overcurrent level signal that is responsive to both the input line voltage and the current drawn through the converter.